6
Dating of Corals and Other Geological
Samples via the Radioactive Disequilibrium
of Uranium and Thorium Isotopes
Norbert Frank and Freya Hemsing
Abstract
U/Th dating methods have become cornerstone tools for
the determination of the age of climate change recorded in
marine and continental carbonates. Here we describe the
theoretical principles and analytical methods along the
example of U/Th dating the aragonite skeletons of
tropical corals. We demonstrate that a precision limiting
factor is built in the dating principle, known as U-series
open system behavior. Moreover, above all the quality of
the samples is crucial for a successful and accurate age
determination. When using well preserved fossil coral
fragments ages provide measures of past sea level,
contribute to the calibration of the radiocarbon time
scale, and allow for the reconstruction of reef accumulation rates, tectonic subsidence, or uplift. We finally
emphasize that U/Th dating also works for secondary
carbonates such as stalagmites, calcareous tuff and
travertine, but the boundary conditions regarding the U
concentration, and the initial U and Th isotopic composition vary wildly and need careful consideration. When
doing so, highest precision and accuracy can Is feasible.
Dating methods based on the radioactive disequilibrium in the
uranium decay series were developed over the last fifty years.
They can be applied to minerals that, at the time of their
formation, incorporate uranium into their crystal lattice, but not
thorium, whose isotope,
230
Th, is a daughter of
234
U (Fig. 6.1).
This is the case for corals which form their aragonitic, calcareous skeletons from elements present in the seawater. They
are used as an example throughout this chapter. The basic
concept is that, in seawater, there is dissolved uranium but
very little thorium, an insoluble element. Therefore, each
crystal of aragonite formed by the coral incorporates only
uranium and not its first-generation daughters which are all
thorium isotopes. Uranium concentration in seawater is very
stable and homogeneous with 3.3 lg of U per liter of water.
The activity ratio (
234
U/
238
U) is also very stable in the ocean
and is slightly above the radioactive equilibrium with a value
of 1.1468 ± 0.0001 in the open ocean (Andersen et al. 2010).
This slight excess of
234
U is due to preferential leaching of this
isotope from rocks during weathering of the continental crust
(Ivanovich and Harmon 1992). Henderson (2002) proposed
that the seawater ratio (
234
U/
238
U) has remained constant for at
least the past 800,000 years. However, moderate variations
of ± 0.01 in this ratio are possible, due to changes in
weathering of the continental crust, sea level changes, and
variations in freshwater runoff from rivers resulting from climate changes (Esat and Yokoyama 2006). Very recently, it has
been suggested that this ratio may also be dependent on the
ocean circulation (Chen et al. 2016).
Let us go back to the coral. If the skeleton of a coral
remains a chemically closed system after its formation (in
other words, without any exchange of uranium or thorium
with its sedimentary environment) the
230 Th from the decay
of
234 U accumulates progressively over time, while the
excess of
234 U decreases.
The state of this radioactive disequilibrium allows for a
very precise determination of the coral age depending on the
measurement technique used. When this dating method was
developed, the isotopes of uranium and thorium were measured by their radioactivity, either directly by a spectrometry
or indirectly by c spectrometry, enabling age determination
ranging from a few thousand years to about 300,000 years.
Now, we are able to measure the abundance of these isotopes by characterizing them according to their masses,
giving a much better accuracy. The age range measurable by
thermal ionization mass spectrometry or even multi-collector
inductively coupled plasma source mass spectrometry now
N. Frank
Laboratoire des Sciences du Climat et de L’Environnement,
LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay,
Gif-Sur-Yvette, 91190, France
N. Frank (&) Á F. Hemsing
Institute of Environmental Physics, Heidelberg University,
Im Neuenheimer Feld 229, 69120 Heidelberg, Germany
e-mail: Norbert.Frank@iup.uni-heidelberg.de
© Springer Nature Switzerland AG 2021
G. Ramstein et al. (eds.), Paleoclimatology, Frontiers in Earth Sciences,
https://doi.org/10.1007/978-3-030-24982-3_6
89
Dating of Corals and Other Geological
Samples via the Radioactive Disequilibrium
of Uranium and Thorium Isotopes
Norbert Frank and Freya Hemsing
Abstract
U/Th dating methods have become cornerstone tools for
the determination of the age of climate change recorded in
marine and continental carbonates. Here we describe the
theoretical principles and analytical methods along the
example of U/Th dating the aragonite skeletons of
tropical corals. We demonstrate that a precision limiting
factor is built in the dating principle, known as U-series
open system behavior. Moreover, above all the quality of
the samples is crucial for a successful and accurate age
determination. When using well preserved fossil coral
fragments ages provide measures of past sea level,
contribute to the calibration of the radiocarbon time
scale, and allow for the reconstruction of reef accumulation rates, tectonic subsidence, or uplift. We finally
emphasize that U/Th dating also works for secondary
carbonates such as stalagmites, calcareous tuff and
travertine, but the boundary conditions regarding the U
concentration, and the initial U and Th isotopic composition vary wildly and need careful consideration. When
doing so, highest precision and accuracy can Is feasible.
Dating methods based on the radioactive disequilibrium in the
uranium decay series were developed over the last fifty years.
They can be applied to minerals that, at the time of their
formation, incorporate uranium into their crystal lattice, but not
thorium, whose isotope,
230
Th, is a daughter of
234
U (Fig. 6.1).
This is the case for corals which form their aragonitic, calcareous skeletons from elements present in the seawater. They
are used as an example throughout this chapter. The basic
concept is that, in seawater, there is dissolved uranium but
very little thorium, an insoluble element. Therefore, each
crystal of aragonite formed by the coral incorporates only
uranium and not its first-generation daughters which are all
thorium isotopes. Uranium concentration in seawater is very
stable and homogeneous with 3.3 lg of U per liter of water.
The activity ratio (
234
U/
238
U) is also very stable in the ocean
and is slightly above the radioactive equilibrium with a value
of 1.1468 ± 0.0001 in the open ocean (Andersen et al. 2010).
This slight excess of
234
U is due to preferential leaching of this
isotope from rocks during weathering of the continental crust
(Ivanovich and Harmon 1992). Henderson (2002) proposed
that the seawater ratio (
234
U/
238
U) has remained constant for at
least the past 800,000 years. However, moderate variations
of ± 0.01 in this ratio are possible, due to changes in
weathering of the continental crust, sea level changes, and
variations in freshwater runoff from rivers resulting from climate changes (Esat and Yokoyama 2006). Very recently, it has
been suggested that this ratio may also be dependent on the
ocean circulation (Chen et al. 2016).
Let us go back to the coral. If the skeleton of a coral
remains a chemically closed system after its formation (in
other words, without any exchange of uranium or thorium
with its sedimentary environment) the
230 Th from the decay
of
234 U accumulates progressively over time, while the
excess of
234 U decreases.
The state of this radioactive disequilibrium allows for a
very precise determination of the coral age depending on the
measurement technique used. When this dating method was
developed, the isotopes of uranium and thorium were measured by their radioactivity, either directly by a spectrometry
or indirectly by c spectrometry, enabling age determination
ranging from a few thousand years to about 300,000 years.
Now, we are able to measure the abundance of these isotopes by characterizing them according to their masses,
giving a much better accuracy. The age range measurable by
thermal ionization mass spectrometry or even multi-collector
inductively coupled plasma source mass spectrometry now
N. Frank
Laboratoire des Sciences du Climat et de L’Environnement,
LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay,
Gif-Sur-Yvette, 91190, France
N. Frank (&) Á F. Hemsing
Institute of Environmental Physics, Heidelberg University,
Im Neuenheimer Feld 229, 69120 Heidelberg, Germany
e-mail: Norbert.Frank@iup.uni-heidelberg.de
© Springer Nature Switzerland AG 2021
G. Ramstein et al. (eds.), Paleoclimatology, Frontiers in Earth Sciences,
https://doi.org/10.1007/978-3-030-24982-3_6
89